A delta sigma modulator is an electronic component configured to output a digital signal to represent an analog input signal. The modulator outputs the digital signal at a logic high value or a logic low value at a relatively high frequency to represent the analog signal. Because the digital signal is highly quantized, the digital signal typically has significant quantization noise. To prevent the quantization noise from adversely affecting the modulator, the modulator is generally designed to shape the frequency response of the quantization noise such that the noise appears outside of a frequency range of interest, i.e., the passband of the modulator.
When used in an analog-to-digital converter (ADC), the digital output of a delta sigma modulator feeds back and is modulated with a reference voltage. Under ideal circumstances, the reference voltage is a constant DC voltage. During the operation of the modulator, however, the reference voltage may pick up electrical interference from other circuitry on the same substrate that is referred to as tones. When the energy of the analog input signal of the modulator is relatively low (e.g., no input signal is applied), the modulator may convolve the tones with out-of-band tones caused by limit cycle oscillations. The convolution may cause the tones to get folded down into spurious tones in the frequency range of interest. The spurious tones may degrade the performance of the modulator. It would be desirable to minimize sensitivity to spurious tones to reduce performance degradation of a delta sigma modulator.